2. Large-scaled excavation work at soft sandy ground with high ground-water level
The construction site of the Niigata Minato tunnel is located on the sandy ground where setting of the ground constant is difficult and ground-water level is high, and thus implementing effective measures was requested for large-scaled excavation work. The taken measures for designing are as follows.
1) Landslide protection designing for large-scaled excavation work made on sandy ground where setting of the ground constant is difficult and ground-water level is high
- For the categorization of designing geological layer, all the layers are assumed to be sand stratum, and the soil constant is set to the minimum value of the mean value of the each layer.
- Excessive pore water pressure is calculated by analyzing the seepage flow and it is considered the reduction of submerged unit weight γ' of the excavation ground by hydraulic gradient and is also considered pore water pressure excessive as external force as the design load
- For the level ground reaction coefficient that is considered to have complex performance and varies depending on the excavation width, the landslide protecting calculation was executed by 2d elasticity FEM analysis that was able to consider the excavation width and evaluated considering the difference of excavation width between the experiment work and temporal main construction work.
- Limited areal hydraulic gradient is calculated by analyzing the seepage flow and the safety of the penetration depth to the limited areal hydraulic gradient is confirmed based on the value of 6.0 that was confirmed by the experiment work.
2) Rational landslide protection designing supported by test construction and measurement construction
- Application of the ground constant based on soil investigation and experiment work
- Application of the designing and installation work result obtained from left bank shaf construction to the temporal designing of the right bank shaft
